Pilots typically fly aircraft according to a flight plan to guide the aircraft from the departure airport to an airport. In the case of commercial aircraft, the flight plan is often prepared by a dispatcher or other non-pilot employee of the commercial aircraft operator. The dispatcher generates a flight plan that best achieves goals of the commercial aircraft operator. Such goals include arriving on time, performing the flight for minimum cost, and performing the flight with maximum possible passenger comfort.
Often, an aircraft flight is not performed exactly as planned. For example, weather and/or other air traffic may cause an aircraft to divert from a planned route, fly slower than planned, and/or fly lower than planned. When such in air deviations occur, it is the pilots' responsibility to determine whether to further modify the flight plan to recover from the deviation and, if so, how to modify the flight plan. For example, a pilot may contemplate deviating around a storm, but the deviation increases the total travel distance for the flight and therefore results in a delayed arrival. The pilot may contemplate increasing speed to avoid the delayed arrival, but such increased speed results in more fuel being burned and, potentially, a more expensive flight. The pilot may also consider not deviating from the flight plan and flying through the weather, but the weather may cause a bumpy, unpleasant flight experience for the passengers.